Adding new fonts in Gimp is super simple. Here's how to do it.
1) Search and download new fonts to your desktop.
2) Unzip your font packages right onto your desktop. You can either unzip the files by right clicking the folder then select unzip from the drop down menu or you can unzip the file from the terminal. If you chose to unzip the file from the terminal, follow these commands:
$ sudo su
# cd
# cd /home/your-login-name/Desktop
# unzip filename.zip
# exit
3) Open these unzip font packages to see the font files. There are about 26 type font extensions but the most popular ones are Windows Font File (.fnt,) Generic Font File (.fon,) OpenType Font (.otf,) and TrueType Font (.ttf.) You want to look for file(s) with these extensions. For example, there are two files in my unzip font folder, fontlicense.txt and robot.ttf. I will copy "robot.ttf" from my desktop to my Gimp's fonts folder and delete the unwanted files/folder.
4) Copy the font files to the Gimp's fonts folder. Your Gimp's fonts folder can be found in /home/your-login-name/.gimp2.x/fonts.
$ sudo su
# cd /home/my-login-name/Desktop
# cp robot.ttf /home/my-login-name/.gimp2.x/fonts
# exit
5) Restart Gimp.